Magnetoresistive memory devices store data based on varying the resistance across the memory device such that a read current through a memory cell in the memory device will result in a voltage drop having a magnitude that is based on the information stored in the memory cell. For example, in certain magnetic memory devices, the voltage drop across a magnetic tunnel junction (MTJ) can be varied based on the relative magnetic states of the magnetoresistive layers within the memory cell. In such memory devices, there is typically a portion of the memory cell that serves as a reference and has a fixed magnetic state. Another portion has a free magnetic state that is controlled to have magnetization either parallel or antiparallel to the fixed magnetic state. Because the resistance of the memory cell changes based on whether the magnetization of the free portion (free layer) is parallel or antiparallel to magnetization of the reference portion (reference layer), information can be stored by setting the magnetic orientation of the free layer. The information is later retrieved by sensing the resistance of the free layer. Such magnetic memory devices are well known in the art.
Writing spin-torque magnetic memory cells can be accomplished by sending a write current through the memory device where the spin angular momentum carried by the current between the reference and free layers can change the magnetic state of the free layer. Depending on the direction of the current through the memory cell (e.g. up or down), the resulting magnetization of the free layer will either be parallel or antiparallel to the reference layer. If the parallel orientation represents a logic “0”, the antiparallel orientation represents a logic “1”, or vice versa. Thus, the direction of write current flowing through the memory device determines whether the memory cell is written to a first state or a second state. Such memory devices are often referred to as spin-transfer torque memory devices (STT-MRAM). In such memories, the magnitude of the write current is typically greater than the magnitude of a read current used to sense the information stored in the memory cells.
Because the magnitude of write current needed to switch the memory cells can be significant, repeated application of the write current through the magnetic tunnel junction can lead to breakdown of the magnetic tunnel junction over time. Therefore, it is desirable to provide techniques for switching the free layers of memory cells in a manner that avoids breakdown issues.